Better AppImage support

[TheAssassin]

There's two major issues with the AppImage support in this application.

The autostart in https://github.com/nextcloud/desktop/blob/master/src/common/utility_unix.cpp asks QApplication for the location of the current binary. However, in an AppImage, this points to a file in a temporary mountpoint. Instead, the AppImage should be registered to be started automatically.

You can check whether you are in an AppImage by checking whether $APPIMAGE is set. $APPIMAGE also contains the path to the AppImage, which you want to use here for Exec.

You should consider adding a TryExec key pointing to the same path. Then, the desktop entry is hidden if the AppImage file gets lost (e.g., because the user deletes or moves it). The obsolete desktop files should be cleaned up from time to time.

The desktop integration code is probably not compatible with AppImage integration software like https://github.com/TheAssassin/AppImageLauncher or https://github.com/AppImage/appimaged. You should consider using libappimage to handle desktop integration. It provides a function to check whether the file is registered already, and provides functions to add and remove integration.

You should also improve the "Am I in an AppImage?" check in https://github.com/nextcloud/desktop/blob/master/src/libsync/configfile.cpp by either checking for $APPIMAGE or $APPDIR. The latter contains the path of the mountpoint, if you should need that for some reason.

CC @rullzer

[pvincent974b]

Same issue with AppImage and autostart.
When I click on 'System Autostart', a new file is created in $HOME/.config/autostart/Nextcloud.desktop.
However it refers to a script located in directory /tmp/.mount_NextclvAg5yF/ which gets removed after reboot.